This invention relates to silica-filled, cured rubber, and more particularly, to cured rubber having improved compression set and a treating method for manufacturing the same.
The compression set of silica-filled rubber largely differs with the amount and type of silica filler, wettability of silica filler with rubber, and other factors. For the purpose of improving the compression set, investigations have heretofore been made on the amount and type of silica filler and a variety of wetters for improving the wettability of silica filler.
Prior art approaches are successful in improving the compression set to some extent, however, better compression sets are required in some applications. There is a desire to have cured rubber having an improved compression set.
It has been found that when a cured rubber made of a curable rubber composition comprising a curable organopoly-siloxane or perfluoropolyether group-containing organosilicon compound, a curing agent therefor, and a silica filler having a mean particle size of 0.001 to 10 xcexcm is post treated by immersing it in a hydrolyzable aminosilane or amidosilane having a Sixe2x80x94N bond, preferably in the presence of a rubber swelling solvent, the cured rubber is significantly improved in compression set.
In a first aspect, the invention provides a cured rubber made of a curable rubber composition comprising a curable organopolysiloxane, a curing agent therefor, and a silica filler having a mean particle size of 0.001 to 10 xcexcm, the cured rubber being post treated with a hydrolyzable aminosilane or amidosilane having a Sixe2x80x94N bond.
In a second aspect, the invention provides a cured rubber made of a curable rubber composition comprising a curable perfluoropolyether group-containing organosilicon compound, a curing agent therefor, and a silica filler having a mean particle size of 0.001 to 10 xcexcm, the cured rubber being post treated with a hydrolyzable aminosilane or amidosilane having a Sixe2x80x94N bond.
In a third aspect, the invention provides a method for treating a cured rubber obtained by curing a curable rubber composition as set forth above. The method involves the step of immersing the cured rubber in a hydrolyzable aminosilane or amidosilane having a Sixe2x80x94N bond in the presence of a rubber swelling solvent.
The cured rubber of the invention is made of either (I) a silicone rubber composition comprising a curable organopolysiloxane, a curing agent therefor, and a silica filler, or (II) a fluororubber composition comprising (A) a curable perfluoropolyether group-containing organosilicon compound, (B) a curing agent therefor, and (C) a silica filler.
In the silicone rubber composition (I), the curable organopolysiloxane is preferably of the following average compositional formula.
RnSiO(4xe2x88x92n)/2 
In the formula, R, which may be the same or different, stands for substituted or unsubstituted monovalent hydrocarbon groups, preferably of 1 to 12 carbon atoms and more preferably 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl and butyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl, allyl, butenyl and hexenyl; aryl groups such as phenyl and tolyl; aralkyl groups such as benzyl and xcex2-phenylpropyl; and substituted ones of the foregoing groups in which some or all of the hydrogen atoms attached to carbon atoms are replaced by halogen atoms, cyano groups or the like, such as chloromethyl, trifluoropropyl and cyanoethyl. The organo-polysiloxane should preferably have at least two alkenyl groups per molecule. Of the R groups, alkenyl groups, especially vinyl groups should preferably account for 0.001 to 10 mol %, especially 0.01 to 5 mol %. The organopolysiloxane is usually end-capped with trimethylsilyl, dimethylvinylsilyl, dimethylhydroxysilyl or trivinylsilyl groups. The letter n is a positive number from 1.98 to 2.02.
The organopolysiloxane preferably has a degree of polymerization of at least about 100, more preferably about 100 to 100,000, and most preferably about 3,000 to 20,000.
Exemplary organopolysiloxanes are given by the structural formulas below. 
The curing agent used herein is selected as appropriate for the curing mechanism of silicone rubber. For the heat vulcanization type, for example, a curing method using an organic peroxide and an addition curing method using an addition reaction curing agent combined with a catalyst are useful. Typical for the room temperature vulcanization type is a condensation curing method using a condensation crosslinker and a condensation reaction promoting catalyst.
The organic peroxide curing method uses organic peroxide curing agents, for example, chlorine-free organic peroxides such as benzoyl peroxide, p-methylbenzoyl peroxide, o-methylbenzoyl peroxide, 2,5-dimethyl-2,5-di-t-butylperoxyhexane, t-butylperoxybenzoate, dicumyl peroxide, and cumyl t-butyl peroxide. Especially for atmospheric hot air vulcanization, acyl organic peroxides such as benzoyl peroxide, p-methylbenzoyl peroxide, and o-methylbenzoyl peroxide are preferred.
These organic peroxides may be used alone or in admixture of two or more. An appropriate amount of the organic peroxide added is about 0.1 to 10 parts, and especially about 0.3 to 5 parts by weight per 100 parts by weight of the organopolysiloxane. Too small an amount of the organic peroxide may provide short crosslinking whereas too large an amount may bring no further increase in cure rate.
In the addition reaction curing method, conventional addition reaction curing agents are used. Often an organo-hydrogenpolysiloxane having at least two, preferably at least three Sixe2x80x94H groups per molecule is used. It is typically represented by the formula:
Rxe2x80x2aHbSiO(4xe2x88x92axe2x88x92b)/2 
wherein Rxe2x80x2 is as defined for R, preferably methyl, phenyl or trifluoropropyl, most preferably methyl, and letters xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d are positive numbers satisfying 0xe2x89xa6a less than 3, 0 less than bxe2x89xa63, and 0 less than a+bxe2x89xa63. Examples are methylhydrogenpolysiloxane and copolymers of methylhydrogenpolysiloxane with dimethyl-polysiloxane. The organohydrogenpolysiloxane used herein usually has less than about 400 silicon atoms per molecule. Typical organohydrogenpolysiloxanes are exemplified by the following formula. 
The organohydrogenpolysiloxane is preferably added in such amounts that 0.5 to 3 mol of Sixe2x80x94H groups are available per mol of alkenyl groups in the organopolysiloxane.
For the rubber to be cured through addition reaction or hydrosilylation reaction, addition reaction promoting catalysts, typically platinum group compounds are preferably used. The platinum group compound serves to promote the addition reaction or hydrosilylation reaction of the curing agent to the polymer backbone. Since noble metal compounds are generally expensive, commercially readily available platinum compounds are often used. Examples include chloroplatinic acid, complexes of chloroplatinic acid with olefins such as ethylene, complexes of chloroplatinic acid with alcohols or vinylsiloxanes, and solid catalysts in the form of platinum on silica, alumina or carbon. To obtain more uniform cured products, a solution of chloroplatinic acid or a complex thereof in a suitable solvent is admixed with the first component prior to use.
Known catalysts in the form of platinum group compounds other than the platinum compounds include rhodium, ruthenium, iridium and palladium compounds, for example, RhCl(PPh3)3, RhCl(CO)(PPh3)2, RhCl(C2H4)2, Ru3(CO)12, IrCl(CO)(PPh3)2, and Pd(PPh3)4.
The amount of the catalyst used is not critical. Partially because of expensiveness, the catalyst is usually used in an amount of about 1 to 1,000 ppm, desirably about 10 to 500 ppm based on the organopolysiloxane.
For the rubber to be cured through condensation reaction, any of acetic acid, alcohol, oxime and acetone type curing agents may be selected so as to comply with a particular curing mechanism. Specifically, suitable condensation reaction curing agents are shown below. 
An appropriate amount of the condensation reaction curing agent used is about 2 to 15 parts by weight per 100 parts by weight of the organopolysiloxane.
For the rubber to be cured through condensation reaction, condensation reaction promoting catalysts are usually added in catalytic amounts. The catalysts include organic tin compounds, titanium compounds, and guanidyl group-containing compounds. Specific examples of the catalyst are shown below. 
Next, reference is made to the fluororubber composition (II). The perfluoropolyether group-containing organosilicon compound used herein is preferably a fluorinated amide compound of the following general formula (1), as disclosed in JP-A 8-198926 of the same assignee as the present invention which is incorporated herein by reference. 
Herein, R1 is a monovalent hydrocarbon group of 1 to 10 carbon atoms selected from among alkyl, cycloalkyl, alkenyl, aryl and aralkyl groups, and substituted ones of the foregoing groups in which some or all of the hydrogen atoms are replaced by halogen atoms. R2 is hydrogen or a monovalent hydrocarbon group as defined for R1. Q is a group of the following general formula (2) or (3). 
Herein, R3 is a divalent hydrocarbon group of 1 to 20 carbon atoms, which may be separated by at least one of oxygen, nitrogen and silicon atoms, selected from among alkylene, cycloalkylene, and arylene groups, substituted ones of the foregoing groups in which some of the hydrogen atoms are replaced by halogen atoms, and combinations of such alkylene groups with arylene groups. 
Herein, each of R4 and R5 is a divalent hydrocarbon group of 1 to 10 carbon atoms selected from among alkylene, cycloalkylene, and substituted ones of the foregoing groups in which some of the hydrogen atoms are replaced by halogen atoms.
Rf is a divalent perfluoroalkylene group of the formula: xe2x80x94CmF2mxe2x80x94 wherein m is 1 to 10, or a divalent perfluoropolyether group selected from groups of the formulas shown below, and the letter xe2x80x9caxe2x80x9d is an integer of 0 to 10. 
Y is F or CF3 group, p, q and k are integers satisfying pxe2x89xa71, qxe2x89xa71, 2xe2x89xa6p+qxe2x89xa6200, and 0xe2x89xa6kxe2x89xa66. 
The letters r, s and t are integers satisfying 0xe2x89xa6rxe2x89xa66, sxe2x89xa70, txe2x89xa70, and 0xe2x89xa6s+txe2x89xa6200. 
Y is F or CF3 group, u and v are integers of 1 to 20.
xe2x80x94CF2CF2(OCF2CF2CF2)wxe2x80x94OCF2CF2xe2x80x94
The letter w is an integer of 1 to 100.
In formula (1), R1 is a monovalent hydrocarbon group of 1 to 10 carbon atoms, especially 1 to 8 carbon atoms, and preferably free of aliphatic unsaturation; and R2 is hydrogen or a monovalent hydrocarbon group like R1. Examples of the monovalent hydrocarbon groups represented by R1 and R2 include alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, octyl and decyl; cycloalkyl groups such as cyclopentyl, cyclohexyl and cycloheptyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl and cyclohexenyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl, phenylpropyl and methylbenzyl; and substituted ones of the foregoing groups in which some or all of the hydrogen atoms are replaced by halogen atoms, typically halo-substituted alkyl groups such as chloromethyl, chloropropyl, bromoethyl, 3,3,3-trifluoropropyl and 6,6,6,5,5,4,4,3,3-nonafluorohexyl.
In formula (1), Q is a group of the general formula (2) or (3). 
In formula (2), R2 is as defined above. R3 is a substituted or unsubstituted divalent hydrocarbon group of 1 to 20 carbon atoms, and especially 2 to 10 carbon atoms, for example, alkylene groups such as methylene, ethylene, propylene, methylethylene, butylene and hexamethylene; cycloalkylene groups such as cyclohexylene; arylene groups phenylene, tolylene, xylylene, naphthylene, and biphenylene; substituted ones of the foregoing groups in which some of the hydrogen atoms are replaced by halogen atoms or the like; and combinations of such substituted or unsubstituted alkylene groups with arylene groups.
R3 may be a group as above in which at least one atom selected from among oxygen, nitrogen and silicon atoms intervenes midway the linkage. In this case, the oxygen atom may intervene in the form of xe2x80x94Oxe2x80x94, and the nitrogen atom may intervene in the form of xe2x80x94NRxe2x80x2xe2x80x94 wherein Rxe2x80x2 is hydrogen or an alkyl group of 1 to 8 carbon atoms, especially 1 to 6 carbon atoms or aryl group. The silicon atom may intervene as a straight or cyclic organosiloxane-containing group or organosilylene group as shown below. 
Herein, Rxe2x80x3 is an alkyl group of 1 to 8 carbon atoms or aryl group as exemplified above for R1 and R2; Rxe2x80x3xe2x80x2 is an alkylene group of 1 to 6 carbon atoms or arylene group as exemplified above for R3, and n is an integer of 0 to 10, and especially 0 to 5.
Illustrative examples of the groups separated by oxygen, nitrogen and silicon atoms are given below. Me is methyl. 
In formula (3), each of R4 and R5 is a divalent hydrocarbon group of 1 to 10 carbon atoms and especially 2 to 6 carbon atoms, for example, alkylene groups such as methylene, ethylene, propylene, methylethylene, butylene, and hexamethylene; cycloalkylene groups such as cyclohexylene; and substituted ones of the foregoing groups in which some of the hydrogen atoms are replaced by halogen atoms.
Illustrative examples of Q in formula (1), represented by formula (2) or (3), are given below. In the following chemical formulas, Me is methyl, Ph is phenyl, Rf is as defined above, and X is hydrogen, methyl or phenyl. 
Illustrative examples of Rf in formula (1) are given below. 
It is noted that in formula (1), xe2x80x9caxe2x80x9d is an integer of 0 to 10, indicating that the fluorinated amide compound of formula (1) contains at least one divalent perfluoroalkylene group or divalent perfluoropolyether group per molecule. Preferably, xe2x80x9caxe2x80x9d is an integer of 1 to 6.
It is appreciated that the fluorinated amide compound of formula (1) can be prepared by the method described in JP-A 8-198926.
Preferably, the fluorinated amide compound of formula (1) has a viscosity in the range of about 100 to 100,000 centistokes at 25xc2x0 C., more preferably about 200 to 20,000 centistokes at 25xc2x0 C.
The curing agent is selected so as to comply with the curing reaction of the perfluoropolyether group-containing organosilicon compound. Usually, an organohydrogen-polysiloxane having at least two, preferably at least three, hydroxyl (Sixe2x80x94H) groups in a molecule is used. Suitable addition reaction crosslinking agents are fluorinated organohydrogensiloxanes containing at least one group selected from among monovalent perfluorooxyalkyl, monovalent perfluoroalkyl, divalent perfluorooxyalkylene and divalent perfluoroalkylene groups, as well as at least two hydrosilyl groups.
The perfluorooxyalkyl, perfluoroalkyl, perfluorooxy-alkylene and perfluoroalkylene groups are exemplified by the groups of the following general formulas. monovalent perfluoroalkyl groups:
CmF2m+1xe2x80x94
Letter m is an integer of 1 to 20, preferably 2 to 10.
divalent perfluoroalkylene groups:
xe2x80x94CmF2mxe2x80x94
Letter m is an integer of 1 to 20, preferably 2 to 10.
monovalent perfluorooxyalkyl groups: 
Letter n is an integer of 1 to 5.
divalent perfluorooxyalkylene groups: 
Letters m and n each are an integer of 1 to 50, and m+n is an integer of 2 to 100.
The fluorinated organohydrogensiloxane may be cyclic, chainlike or three-dimensional network. Preferred organohydrogensiloxane has in a molecule at least one organic group containing a perfluoroalkyl, monovalent perfluoroalkyl ether group or perfluoroalkylene group or divalent perfluoroalkyl ether group as represented by the following general formulas as the substituent attached to a silicon atom. 
In the above formulas, R6 is a divalent hydrocarbon group of 1 to 10 carbon atoms and especially 2 to 6 carbon atoms, for example, alkylene groups such as methylene, ethylene, propylene, methylethylene, tetramethylene, and hexamethylene; and arylene groups such as phenylene. R7 is hydrogen or a monovalent hydrocarbon group of 1 to 8 carbon atoms and especially 1 to 6 carbon atoms, like R2. Rf1 and Rf2 are independently monovalent perfluoroalkyl, monovalent perfluorooxyalkyl, divalent perfluorooxyalkylene or divalent perfluoroalkylene groups.
In the fluorinated organohydrogensiloxane as component (B), the substituents attached to silicon atoms other than the monovalent organic group having a mono- or divalent fluorinated substituent, that is, perfluoroalkyl, perfluorooxyalkyl, perfluorooxyalkylene or perfluoroalkylene group include monovalent hydrocarbon groups of 1 to 10 carbon atoms, especially 1 to 8 carbon atoms, free of aliphatic unsaturation, as defined for R2. The number of silicon atoms in the molecule of the fluorinated organohydrogensiloxane is not critical although it desirably has about 2 to about 60 silicon atoms, more desirably about 4 to about 60 silicon atoms, and especially about 4 to about 30 silicon atoms.
Illustrative, non-limiting, examples of the fluorinated organohydrogensiloxane are given below. 
The fluorinated organohydrogensiloxane (B) is preferably blended in such amounts that 0.5 to 5 mol, and more preferably 1 to 2 mol of hydrosilyl (Sixe2x80x94H) groups in (B) are available per mol of aliphatic unsaturated groups including alkenyl and cycloalkenyl groups in the perfluoropolyether group-containing compound (A). Less amounts of component (B) may achieve an insufficient degree of crosslinking. Excessive amounts of component (B) may allow chain lengthening to become preferential, inviting undercure, foaming, and losses of heat resistance and other properties. An appropriate amount of component (B) blended is 0.1 to 50 parts by weight per 100 parts by weight of component (A).
Along with the addition reaction crosslinking agents, addition reaction promoting catalysts such as platinum group compounds as previously described are blended in catalytic amounts if necessary.
The silica filler used herein has a mean particle size of 0.001 to 10 xcexcm, and preferably 0.01 to 1.0 xcexcm. Preferably the silica filler has a specific surface area of at least 50 m2/g, and especially 100 to 500 m2/g as measured by the BET method.
The silica filler may be selected from commonly used reinforcing silica (inclusive of dry silica and wet silica) and non-reinforcing silica. Examples of useful silica include fumed silica obtained by burning of silicon tetrachloride, hydrolytic silica obtained by hydrolysis of silicon tetrachloride, silica flour obtained by grinding naturally occurring quartz, spherical silica obtained by melting of quartz, and surface treated silica obtained by surface treating the respective silicas with chlorosilane, silazane, etc.
An appropriate amount of the silica filler blended is about 5 to 200 parts, and especially about 10 to 100 parts by weight per 100 parts by weight of the composition. With too less an amount of silica, the cured rubber may have a low tensile strength. Too large an amount of silica may reduce the elongation of the cured rubber.
According to the invention, the curable composition comprising the polymer, curing agent therefor, and silica filler described above is cured under suitable conditions for its curing reaction to form a cured rubber, which is post treated with a hydrolyzable aminosilane or hydrolyzable amidosilane having a Sixe2x80x94N bond.
The curing conditions for the curable composition are not critical as long as a sufficient amount of heat to induce decomposition of the curing agent and vulcanization of rubber is applicable; and the molding method is not critical and usually selected, for example, from extrusion molding concomitant with continuous vulcanization, press molding and injection molding. The curing conditions are selected in accordance with the molding method and usually include a temperature of about 80 to 200xc2x0 C. and a time of about 5 seconds to 30 minutes. If necessary, this is followed by secondary vulcanization at about 150 to 250xc2x0 C. for about 1 to 10 hours.
The hydrolyzable aminosilane and amidosilane each having a Sixe2x80x94N bond with which the cured rubber is treated are not critical and may have various structures including straight, branched and cyclic structures. Illustrative, non-limiting, examples are given below. Vi is vinyl. 
An appropriate amount of the hydrolyzable aminosilane or amidosilane used is about 0.1 to 2 g, especially about 0.05 to 1 g per gram of the cured rubber. Too small an amount of the silane relative to the cured rubber may fail to achieve sufficient treatment to improve compression set. Too large an amount of the silane is wasteful.
In the practice of the invention, the cured rubber is post treated, for example, by dissolving the hydrolyzable aminosilane or amidosilane in a suitable solvent to form a treating solution, and immersing the cured rubber in the treating solution. The solvent used herein is preferably the one with which the rubber to be treated swells, so that the cured rubber may be treated in the swollen state. Hydrocarbon solvents such as toluene and hexane are effective when the polymer used is a diorganopolysiloxane. Fluorochemical solvents such as m-xylene hexafluoride and perfluorobutyl tetrahydrofuran are useful when the polymer used is a perfluoropolyether polymer.
Treating conditions may be properly selected. Preferred conditions include a treating temperature of about 20 to 120xc2x0 C., especially about 60 to 100xc2x0 C. and a treating time of about 1 to 48 hours, especially about 2 to 24 hours. At the end of treatment, the solvent is removed under suitable conditions, yielding the end cured rubber as treated.
The cured rubber as treated is significantly improved in compression set so that it is applicable to a variety of uses, for example, as O-rings and square rings in fuel, lubricant and operating fluid systems of automobiles and aircraft, O-rings in semiconductor devices, rolls in copiers, molded rubber parts for sealing purposes, and diaphragms. With the treating method of the invention, cured rubber parts having superior cured properties are manufactured in an efficient manner.